VCT 38xxA
ADVANCE INFORMATION
2.4.2. Demodulator
2.4.4. Frequency Demodulator
The entire signal (which might still contain luma) is
now quadrature-mixed to the baseband. The mixing
frequency is equal to the subcarrier for PAL and NTSC,
thus achieving the chroma demodulation. For SECAM,
the mixing frequency is 4.286 MHz giving the quadra-
ture baseband components of the FM modulated
chroma. After the mixer, a low-pass filter selects the
chroma components; a downsampling stage converts
the color difference signals to a multiplexed half-rate
data stream.
The frequency demodulator for demodulating the
SECAM signal is implemented as a CORDIC structure.
It calculates the phase and magnitude of the quadra-
ture components by coordinate rotation.
The phase output of the CORDIC processor is differ-
entiated to obtain the demodulated frequency. After
the deemphasis filter, the Dr and Db signals are scaled
to standard CrCb amplitudes and fed to the crossover
switch.
The subcarrier frequency in the demodulator is gener-
ated by direct digital synthesis; therefore, substan-
dards such as PAL 3.58 or NTSC 4.43 can also be
demodulated.
2.4.5. Burst Detection / Saturation Control
In the PAL/NTSC-system the burst is the reference for
the color signal. The phase and magnitude outputs of
the CORDIC are gated with the color key and used for
controlling the phase-locked-loop (APC) of the demod-
ulator and the automatic color control (ACC) in PAL/
NTSC.
2.4.3. Chrominance Filter
The demodulation is followed by a low-pass filter for
the color difference signals for PAL/NTSC. SECAM
requires a modified low-pass function with bell-filter
characteristic. At the output of the low-pass filter, all
luma information is eliminated.
The ACC has a control range of +30...−6 dB.
Color saturation can be selected once for all color
standards. In PAL/NTSC it is used as reference for the
ACC. In SECAM the necessary gains are calculated
automatically.
The low-pass filters are calculated in time multiplex for
the two color signals. Three bandwidth settings (nar-
row, normal, broad) are available for each standard.
For PAL/NTSC, a wide band chroma filter can be
selected. This filter is intended for high bandwidth
chroma signals, e.g. a non-standard wide bandwidth
S-VHS signal.
For SECAM decoding, the frequency of the burst is
measured. Thus, the current chroma carrier frequency
can be identified and is used to control the SECAM
processing. The burst measurements also control the
color killer operation; they are used for automatic stan-
dard detection as well.
2.4.6. Color Killer Operation
The color killer uses the burst-phase/burst-frequency
measurement to identify a PAL/NTSC or SECAM color
signal. For PAL/NTSC, the color is switched off (killed)
as long as the color subcarrier PLL is not locked. For
SECAM, the killer is controlled by the toggle of the
burst frequency. The burst amplitude measurement is
used to switch-off the color if the burst amplitude is
below a programmable threshold. Thus, color will be
killed for very noisy signals. The color amplitude killer
has a programmable hysteresis.
PAL/NTSC
SECAM
Fig. 2–5: Frequency response of chroma filters
14
Micronas
MICRONAS [ MICRONAS ]
